1. Field of the Invention
The present invention relates generally to the nitriding of alloys, and particularly to a method of nitriding nickel-chromium-based superalloys using gas-assisted laser nitriding.
2. Description of the Related Art
Inconel® refers to a family of austenitic nickel-chromium-based superalloys produced by the Special Metals Corporation of New Hartford, N.Y. A “superalloy”, or high-performance alloy, is an alloy that exhibits excellent mechanical strength and creep resistance at high temperatures, along with good surface stability, and corrosion and oxidation resistance. Superalloys typically have a matrix with an austenitic face-centered cubic crystal structure. A superalloy's base alloying element is typically nickel, cobalt, or nickel-iron. Superalloy development has relied heavily on both chemical and process innovations and has been driven primarily by the aerospace and power industries. Typical applications are in the aerospace, industrial gas turbine and marine turbine industry, e.g. for turbine blades for hot sections of jet engines.
Inconel® alloys are typically used in high temperature applications. Different Inconel® alloys have widely varying compositions, but all are predominantly nickel, with chromium as the second element. Table 1, below, shows the composition of three such superalloys, Inconel® 600, Inconel® 625 and Inconel® 718:
TABLE IElement (% by mass)Inconel ®Inconel ®Inconel ®Element600625718Ni72.0  58.0  50.0-55.0Cr14.0-17.020.0-23.017.0-21.0Fe 6.0-10.05.0 BalanceMo 8.0-10.02.8-3.3Nb3.15-4.154.75-5.5 Co1.0 1.0 Mn1.0 0.5 0.35 Cu0.5 0.2-0.8Al0.4 0.65-1.15Ti0.4 0.3 Si0.5 0.5 0.35 C0.15 0.1 0.08 S0.0150.0150.015P0.0150.015B0.006
Inconel® alloys are oxidation and corrosion resistant materials that are well suited for service in extreme environments. When heated, Inconel® alloys form a thick, stable, passivating oxide layer protecting the surface from further attack. Inconel® alloys retains strength over a wide temperature range, which is attractive for high temperature applications where aluminum and steel would succumb to creep as a result of thermally induced crystal vacancies. The Inconel® alloys' high temperature strength is developed by solid solution strengthening or precipitation strengthening, depending on the particular alloy. In age hardening or precipitation strengthening varieties, small amounts of niobium combine with nickel to form the intermetallic compound Ni3Nb or “gamma prime” (γ′). Gamma prime forms small cubic crystals that inhibit slip and creep effectively at elevated temperatures.
Inconel® alloys are difficult metals to shape and machine using traditional techniques due to their rapid work hardening. After the first machining pass, work hardening tends to plastically deform either the workpiece or the tool on subsequent passes. For this reason, age-hardened alloys, such as Inconel® 718, are machined using an aggressive but slow cut with a hard tool, minimizing the number of passes required. Alternatively, the majority of the machining can be performed with the workpiece in a solutionised form, with only the final steps being performed after age-hardening. External threads are machined using a lathe to “single point” the threads, or by rolling the threads using a screw machine. Holes with internal threads are made by welding or brazing threaded inserts made of stainless steel. Cutting of plate is often done with a waterjet cutter. Internal threads can also be cut by single point method on lathe, or by threadmilling on a machining center. New whisker reinforced ceramic cutters are also used to machine nickel alloys. They remove material at a rate typically eight times faster than carbide cutters. Inconel® 718 can also be roll threaded after full aging by using induction heat to 1300° F. without increasing grain size.
Nitriding is a heat-treating process that alloys nitrogen into the surface of a metal to create a case hardened surface. It is predominantly used on steel, but also titanium, aluminum and molybdenum. It would be desirable to provide the benefits of nitriding an alloy surface to a superalloy, such as the Inconel° alloys. The three main methods of nitriding typically used in industry are: gas nitriding, salt bath nitriding, and plasma nitriding. In gas nitriding, the donor is a nitrogen rich gas, such as ammonia (NH3). When ammonia comes into contact with the heated work piece, it disassociates into nitrogen and hydrogen. The nitrogen then diffuses from the surface into the core of the material. Recent developments have lead to a process that can be accurately controlled. The thickness and phase constitution of the resulting nitriding layers can be selected, and the process can be optimized for the particular properties required.
In salt bath nitriding, the nitrogen-donating medium is a nitrogen containing salt, such as cyanide salt. The salts used also donate carbon to the workpiece surface, making salt bath a nitrocarburizing process. Plasma nitriding (also known as ion nitriding, plasma ion nitriding or glow-discharge nitriding) is an industrial surface hardening treatment for metallic materials. Typically, an ionized gas, such as nitrogen in a low-pressure regime, is reactive with the surface components of the workpiece. Plasmas, however, are not easy to work with or apply. Conventional gas nitriding and salt bath nitriding are not particularly effective on a superalloy, such as one of the Inconel® superalloys. It would be desirable to provide the benefits of a nitride surface to a nickel-chromium-based superalloy, such as one of the Inconel® superalloys, without the expense or difficulty of working with conventional nitrogen plasmas.
Inconel® 718, in particular, is widely used in the power industry, due to its high resistance to harsh environmental conditions. Niobium segregation, however, results in large scattering of the mechanical properties in the surface region of plates and other structures formed from Inconel® 718. Segregation is typically avoided through the formation of fine structures on the surface through laser-controlled melting. Although Inconel® 718 on its own is somewhat resistant to oxidation, the use of an assisting gas in the laser treatment process is typically deemed to be necessary in order to prevent the formation of oxide species at elevated temperatures in the laser-irradiated region.
As noted above, the assisting gas is typically an inert gas, such as atomic nitrogen (often from dissociated ammonia at high temperature, where the metallic surface acts as a catalyst in the uptake of nitrogen), which prevents the high temperature exothermic oxidation reactions at the surface. In addition to preventing oxidation, nitrogen acts in the formation of nitride species in the laser-irradiated region during the heating process.
Thus, a method of nitriding nickel-chromium-based superalloys solving the aforementioned problems is desired.